Vegetation helped save earth from runaway cooling that would have encased the planet in ice, according to a new study.

The study, which appears in the journal Nature, sheds light on the natural mechanisms that over hundreds of millions of years have swung the globe like a pendulum between deep chill and intense heat.

Around 50 million years ago, the planet's poles were ice-free and crocodiles roamed the Arctic.

But that was followed by a long period of cooling, in which levels of carbon dioxide (CO2), the principal 'greenhouse' gas that traps solar heat, progressively declined.

Belching volcanoes provided the main source of this CO2 - in contrast to today, when the gas comes overwhelmingly from burning fossil fuels.

But there was also a force which removed CO2: a chemical reaction that occurs when silica rocks are weathered.

Over time, the gas is dissolved into groundwater, which flows to the sea and eventually the carbon is sequestered on the ocean floor.

Climate scientists have long puzzled about what happened at a key point in this weathering process.

Around 25 million years ago, earth was wrenched by a period of mountain building that threw up the Himalayas and the Andes.

This created conditions that, in theory, should have sucked nearly all the CO2 out of the atmosphere and plunged the planet into a deep freeze.

Yet it clearly did not happen, and the question is why.

Plant buffer

The answer, according to US geophysicists, lies in the buffering power of plants.

Vegetation, especially trees, suck in atmospheric CO2 in the process of photosynthesis and also play a key role in the weathering of rocks.

Their roots secrete acids that dissolve minerals, hold soils and increase the amount of CO2 dissolved in groundwater.

As the CO2 levels plummeted, plants were starved of their essential gas for life, according to the team's hypothesis.

This slowed the weathering process down, and led to less burial of the carbon. As a result, there remained enough CO2 in the air to avoid the 'iceball earth' scenario.

"As the CO2 concentration of earth's atmosphere decreased to about 200 to 250 parts per million (ppm), CO2 levels stabilised," says lead author Mark Pagani, an associate professor of geology and geophysics at Yale University.

The study is based on simulations of the global carbon cycle and observations from plant growth experiments.

Modern times

If plants saved earth from endless chill, they are unlikely to do the same when it comes to human-induced warming, say the authors.

CO2 levels in the atmosphere today are around 385 ppm, compared with 280 ppm before the Industrial Revolution.

"We are releasing CO2 to the atmosphere about 100 times faster than all the volcanoes in the world put together," says Dr Ken Caldeira of the Carnegie Institution for Science.

"While these weathering processes will eventually remove the CO2 we are adding to the atmosphere, they act too slowly to help us avoid dangerous climate change.

"It will take hundreds of thousands of years for these rock-weathering processes to remove our fossil-fuel emissions from the atmosphere."